1. Field of the Invention
The present invention relates to, for example, a semiconductor light-emitting device and a method of manufacturing the same, particularly, to a semiconductor light-emitting device using an InGaAlP material and a method of manufacturing the same.
2. Description of Related Art
A semiconductor light-emitting device such as an LED (light-emitting diode) comprises a light-emitting layer, and light is emitted from the light-emitting layer in accordance with the voltage applied from the electrodes on both sides of the light-emitting device. In order to improve the light-emitting efficiency of the light-emitting device, it is necessary to prevent the light emitted from the light-emitting layer from being reflected and absorbed within the device.
In general, an n-type GaAs is used as a substrate of an LED using an InGaAlP series material.
FIG. 15 shows a first prior art of a semiconductor light-emitting device using the material noted above. As shown in the drawing, a buffer layer 22 is formed on a GaAs substrate 21, and a light reflecting layer 23 is formed on the buffer layer 22. Also, a light-emitting layer comprising an n-cladding layer 24, an active layer 25 and a p-cladding layer 26 is formed on the light reflecting layer 23. Further, a p-GaAlAs current diffusion layer 29 is formed on the light-emitting layer 27.
It should be noted that the GaAs substrate 21 is not transparent to a visible light and, thus, the light emitted from the light-emitting layer and running downward is absorbed entirely by the GaAs substrate 21. This is a serious obstacle to the improvement in the brightness of the LED.
Such being the situation, proposed is a method of using a GaP substrate as the substrate of the semiconductor light-emitting device. FIG. 16 shows a second prior art of a semiconductor light-emitting device. In the second prior art, a light-emitting layer 27 is formed by a MOCVD method (Metal Organic Chemical Vapor Deposition method) on a GaAs substrate (not shown), followed by forming a thick p-type GaP layer 30 by an HVPE method (Hydride Vapor Phase Epitaxy method) having a thickness of 50 μm on the light-emitting layer 27, as shown in FIG. 16. Further, the GaAs substrate is removed, and an n-type GaP substrate 28 transparent to a visible light is bonded to the light-emitting layer 27 in place of the n-type GaAs substrate. In the semiconductor light-emitting device of the particular construction, the light emitted from the light-emitting layer 27 is taken out upward, downward, rightward and leftward, i.e., in every direction. It follows that it is possible to obtain the brightness of the light emission 2 to 3 times as high as that in the first prior art.
It should be noted, however, that, in bonding the GaP substrate 28 to the light-emitting layer 27 in the light-emitting device of the construction shown in FIG. 16, it is necessary to apply a heat treatment at a temperature higher than the heat treating temperature for the MOCVD step (about 700° C.). It follows that the light-emitting layer 27 receives a thermal damage in the bonding process of the GaP substrate 28. Particularly, where zinc is used as the p-type impurity of the p-cladding layer 26, zinc is diffused in a large amount into the active layer 25 in the step of the heat treatment at a high temperature so as to deteriorate the crystallinity of the active layer 25. As a result, the power of the light emitted from the light-emitting layer 27 included in the second prior art is rendered markedly inferior to that in the first prior art. It follows that the brightness in the second prior art fails to reach a level that is 2 times as high as that in the first prior art.
Under the circumstances, it is conceivable to lower the heat treating temperature in the bonding step in order to avoid the damage done to the light-emitting layer by the heat. In this method, however, a satisfactory ohmic contact fails to be formed at the bonding interface between the n-cladding layer 25 and the GaP substrate 28, resulting in elevation of the operating voltage of the light-emitting device.
FIG. 17 is a graph showing the relationship between the heat treating temperature in the bonding step and the relative light output of the device and the relationship between the heat treating temperature in the bonding step and the operating voltage. In the graph of FIG. 17, the relationship between the bonding temperature and the relative light output is denoted by a solid line, and the relationship between the bonding temperature and the operating voltage is denoted by a broken line. As apparent from the broken line given in FIG. 17, the operating voltage is lowered with increase in the heat treating temperature in the bonding step. It should be noted that a satisfactory ohmic contact can be obtained at about 800° C. On the other hand, the light output of the device is lowered with increase in the heat treating temperature as apparent from the solid line given in FIG. 17. It follows that, in order to obtain a reasonable level of the light output of the device and to lower the operating voltage of the device, it is necessary to select the heat treating temperature in the bonding step falling within an appropriate temperature range. The appropriate temperature range is very narrow (about 790° C. to 810° C.), leading to the problems that it is impossible to obtain a sufficient effect of improving the light output, which is to be obtained by the bonding of the transparent GaP substrate 28, and that it is difficult to produce the semiconductor light emitting device stably with a high yield.